In recent years, as semiconductor ICs (LSIs) have been used for electronic equipment with higher densities and higher levels of integration, the electrode terminals of LSI chips have rapidly increased in the number of pins with narrower pitches. For the mounting of the LSI chips on a wiring substrate, flip-chip mounting has been widely used to reduce a wiring delay. In flip-chip mounting, solder bumps are formed on the electrode terminals of LSI chips and the LSI chips are collectively joined via the solder bumps to electrodes formed on a wiring substrate.
In the prior art, plating and screen printing have been developed as techniques for forming bumps. Plating is suitable for narrow pitches but results in a complicated process and low productivity. Screen printing can achieve high productivity but is not suitable for narrow pitches because a mask is used.
Under these circumstances, some techniques for selectively forming solder bumps on LSI chips or the electrodes of a wiring substrate have been developed in recent years. Since these techniques are suitable for forming small bumps and can collectively form solder bumps, these techniques have achieved high productivity and received attention as techniques applicable to mounting on wiring substrates of next-generation LSIs.
One of the techniques is called solder paste method (for example, see Japanese Patent Laid-Open No. 2000-94179). In this technique, solder paste which is a mixture of solder powder and flux is applied over a substrate on which electrodes have been formed, and then the solder powder is melted by heating the substrate, so that solder bumps are selectively formed on the electrodes having high wettability.
Further, in a technique called super solder method (for example, see Japanese Patent Laid-Open No. 1-157796) a paste composition (chemical reaction deposited solder) mainly composed of organic acid lead salt and metallic tin is applied over a substrate on which electrodes have been formed, and then substitution is performed on Pb and Sn by heating the substrate, so that a Pb—Sn alloy is selectively deposited on the electrodes of the substrate.
Moreover, in a technique called Super Juffit method (for example, see Japanese Patent Laid-Open No. 7-74459), a substrate on which electrodes have been formed is immersed into an agent to form an adhesive coating only on the surfaces of the electrodes, and then solder powder is brought into contact with the adhesive coating to bond the solder powder onto the electrodes. After that, molten solder is selectively formed on the electrodes by heating the substrate.